The present invention relates to an improved transfer roller system for an electrostatic reproduction machine. More particularly, the invention is directed to apparatus for biasing a transfer roller into contact with a photoreceptor with a force which does not vary during operation of the machine.
In conventional xerography, a xerographic plate or photoreceptor comprising a layer of photosensitive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas where light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to an "toner". Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. The developed image is then transferred to a suitable transfer member (e.g., paper), and the image is affixed thereto to form a permanent record of the original document. Residual toner is then removed from the photosensitive surface prior to charging the surface again.
Where an electrically biased transfer roller is used to effect transfer of the developed image from the photoreceptor to the transfer member, problems often arise because of the force which biases the roller into contact with the photoreceptor.
One of the problems that arise in transfer roller systems is defects in copy quality, e.g., "hollow characters" which may result from a sheet of paper being subjected to an excessive pressure as it passes between the transfer roller and the photosensitive surface during the transfer step.
Another copy quality problem which sometimes arises in transfer roller systems relates to undesired nonuniform densities in the transferred image. This results from variations in pressure caused by variations in the dimensions of the photoreceptor or transfer roll diameter, or disturbances within the reproduction machine.